Certain low-rank coals, especially brown coals, lignites and subbituminous coals, hereinafter referred to as low-rank coals or low-rank solid fossil fuels, or both, which generally have a water content of 20% or more by weight, are increasingly of interest in electric-power generation.
When such coals are burned, the flue gas from the combustion chamber generally contains relatively large amounts of sulfur dioxide and, to a far less extent sulfur trioxide, i.e. so-called sulfur emissions in the flue gas.
The low-rank coal with which the present invention is concerned can be distinguished from bituminous or high-rank coal in the sense that it is a younger coal in an earlier form of carbonization.
While low rank coals have been variously characterized e.g. in Standard Specification for Classification of Coals by Rank, Designation D 388-66, 5 pp ASTM, Philadelphia, Pa., the expression as used herein is intended to refer to all low rank solid fossil fuels having a water content of at least 20% by weight.
Such coals may contain from 0.2 to more than 0.9% by weight sulfur which, upon combustion in a boiler, e.g. a power-plant boiler, can be transformed to sulfur dioxide and sulfur trioxide.
It can be observed, more generally, that all fossil fuels naturally contain sulfur which, upon combustion, is converted into its oxides, mainly sulfur dioxide but also some sulfur trioxide.
In the industrial combustion of fossil fuels, it is important that the concentrations of sulfur dioxide and sulfur trioxide in the flue gas (more generally sulfur emissions) do not exceed a predetermined value. The standards are generally established by law but it can be said with accuracy that the smaller the sulfur emissions, the safer the discharge of the flue gases into the atmosphere will be.
One possibility is the reduction of the sulfur content of the fuel. When a significant reduction of the sulfur content of the fuel is not possible, one can achieve a partial desulfurization of the flue gas by special treatment of the latter prior to discharging into the atmosphere.
Such special treatments include scrubbing, chemical treatment of the gas and the like.
The economies of such waste-gas desulfurization systems, where they are based on the principle of adsorption or catalysis, are poor because they require relatively large apparatuses, are not able to compensate for significant fluctuations in the sulfur oxide concentrations in the flue gas, and frequently are contaminated or poisoned by other materials which may be present.
Pretreatment systems have likewise been found to be uneconomical for many fuels, including low-rank coals.
In this period of a world-wide shortage of fuels, the poor economies of pretreatment of the fuel or post-treatment of the flue gas to eliminate sulfur have prevented a significant exploitation of low-calorie solid fuels.
In practice it is found that desulfurization systems for the removal of sulfur dioxide and sulfur trioxide from flue gas are more expensive and least economical for the treatment of those flue gases which result from the combustion of low-rank coals.
For completeness mention may be made of the fact that bituminous coal also contains significant quantities of sulfur and that efforts have been made to reduce the sulfur emissions from the combustion of bituminous coal by the blowing of finely divided calcium oxide or calcium-oxide-containing dry materials as additive into the combustion chamber and/or the convective section of an industrial boiler using bituminous coal. However, sulfur dioxide does not readily react with finely divided calcium oxide introduced in this manner so that the practice has not proved to be practicable for large boiler installations.